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The second refrigerant in the display cooler (2)
author: supermarket refrigerated showcase category: commercial refrigeration Views: 558Time: 2016-08-22
The second refrigerant in the display cooler
inorganic salt aqueous solution
The inorganic salt aqueous solution in the display cooler has a low freezing temperature, which is suitable for cooling in medium and low temperature refrigeration devices. The most widely used is calcium chloride (CaCl2) aqueous solution, as well as sodium chloride (NaCl) and magnesium chloride (MgCI2) aqueous solutions. The relationship between the state of the brine solution and the temperature T and the mass fraction of salt ω. Curve WE is the ice precipitation line, EG is the salt precipitation line, and point E is the eutectic point. The temperature TE corresponding to the eutectic point and the mass fraction of salt WE are called eutectic temperature and eutectic mass fraction, respectively. When the temperature of the solution decreases and the phase change occurs, it is related to the mass fraction of the salt. When four ω WE, the solution is cooled and solidified before the salt is precipitated, and the salt precipitation temperature increases with the increase of the mass fraction of salt. The eutectic temperature is the lowest temperature at which the solution does not undergo ice or salt precipitation.
When preparing the salt solution refrigerant, the mass fraction of salt should not exceed that of the eutectic point. Otherwise, the salt consumption will increase, the specific gravity of the solution will increase, the resistance and the pump power will increase, and the freezing temperature of the cold liquid will increase instead. When preparing the solution, it is only necessary to satisfy the requirement that the ice precipitation temperature is 5-8°C lower than the evaporation temperature of the refrigerant. The eutectic temperatures of CaCI2, NaCl and MgCIz aqueous solutions are 55C, 21C and 34C, respectively. The specific gravity and specific heat capacity of the
salt solution are relatively large. Therefore, the volume circulation of the salt solution required to transfer a certain amount of cooling in the display cooler is small. The salt solution is corrosive, especially the dilute salt solution that is slightly acidic and in contact with air is very corrosive to metal materials. To this end, certain corrosion inhibition measures need to be taken. The brine solution can be made more concentrated to avoid the brine oxidation due to the ventilation of the brine pool; the return port of the refrigerant returning to the brine pool should be set below the liquid level. In addition, add a corrosion inhibitor to the salt solution to make the solution neutral (pH adjusted to about 7.0). Corrosion inhibitors usually use sodium chromate dihydrate (NazCrzO7.2Hzo) solution (the content is 1.5-2.Og/l). Salt water is highly hygroscopic. Due to moisture absorption, the mass fraction of salt gradually becomes stricter and Kapen intends to check regularly.
(3) Organic refrigerant hasThere are many airborne refrigerants, here are just a few examples.
①Methanol (CH30H), ethanol (C2H60H) and their aqueous solutions. The freezing point of methanol is -97°C, and the freezing point of ethanol is -117°C, which can be cooled at lower temperatures. Methanol is slightly more viscous than ethanol in water, and their fluidity is better. Both methanol and ethanol are volatile and flammable. When using them, pay attention to fire prevention. When the machine stops running and the system is at room temperature, be more careful.
②Aqueous solutions of ethylene glycol, propylene glycol and glycerol. Glycerol (glycerin) is a very stable compound, its aqueous solution is non-corrosive to metals, non-toxic, and can be in direct contact with food. The characteristics of the aqueous solutions of ethylene glycol and propylene glycol are similar. Their eutectic temperature reaches -60°C, their specific gravity and specific heat capacity are large, and the solution viscosity is high, which is slightly toxic, but not harmful.
③Pure organic liquid. Pure organic liquids, such as dichloromethane R30 (CHzClz), trichloroethylene R1120 (C2HC13) and other freon liquids, have a very low freezing point (about 100°C or lower); they have a large specific volume, a small viscosity and a small specific heat capacity , Can make the display cooler get a lower cooling temperature.
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